Chaotic signals are random-like signals generated by deterministic processes. The wideband nature of chaotic signals makes them naturally suitable for carrying information in a spread-spectrum communication system environment, and has the advantages of providing an improved capability of anti-jamming, a lowered probability of interception, and an increased resistance to multi-path effects. Furthermore, because of the random-like nature of chaotic signals, their use in communication offers a basic capability of resistance to detection by unintended receivers. Further, chaotic signals offer intrinsic security because of their random-like nature.
Digital communication schemes using chaotic signals as carriers can be broadly classified into two categories. In the first category, the chaotic signals carrying the information have to be synchronously regenerated at the receiver. The recovery of the information thus relies on a process that achieves synchronization of two chaotic signals or systems. There are numerous ways to achieve synchronization, and some specific methods have been disclosed in U.S. Pat. No. 6,363,153 to Parker, et al., U.S. Pat. No. 6,216,093 to Corron, et al., U.S. Pat. No. 6,212,239 to Hayes, U.S. Pat. No. 6,049,614 to Kim, U.S. Pat. No. 5,930,364 to Kim, U.S. Pat. No. 5,923,760 to Abarbanel, et al., and U.S. Pat. No. 5,291,555 to Cuomo, et al. However, methods requiring regeneration of synchronized chaotic signals at the receiver or synchronization of chaotic signals are applicable only in communication systems where the level of additive noise is low, which may not be applicable to a practical environment.
In the second category, no synchronous regeneration of the carrying chaotic signals is required in the receiver. In order to demodulate the received signal, the receiver may rely on a specific structure of each bit which has been configured by the transmitter to facilitate demodulation. A widely known method of this kind is the differential chaos shift keying method, as described in the original paper by Kolumban, et al. in 1996 (“Differential chaos shift keying: a robust coding for chaos communication” published in the Proceedings of 1996 International Workshop on Nonlinear Dynamics of Electronic Systems, pp. 97-92.) However, such a method makes no use of the chaotic properties of the signals and may not be able to resist unintended reception because the fabricated bit structure can be relatively easily detected.
The properties of chaotic signals generated from deterministic processes depend on the types of chaotic signals and the parameters that are used to generate them. Therefore, it is conceptually possible to make use of the built-in properties of chaotic signals for communication. A prior disclosure that exploits built-in properties of chaotic systems is described in U.S. Pat. No. 5,857,165 issued to Corron, et al. However, the method of Corron, et al., as disclosed in U.S. Pat. No. 5,857,165, does not employ any built-in property as a signature for identifying chaotic signals, but makes use of a parameter to achieve synchronization. In this sense, the method by Corron, et al. as disclosed in U.S. Pat. No. 5,857,165 should be considered as a method under the first category.
In brief, current methods relying on synchronizing the generation of chaotic signals between the transmitter and the receiver may not be practicable due to noise addition. However, methods utilizing no synchronization make the transmission vulnerable to interception.